I have recently discovered how to clean up audio using the freeware software Audacity and I thought I would share it.
This might not be rocket science to anyone else, but it took me a while to discover.
When I say this I should point out that it is versions of Audacity before version 2.1.0 that I am referring to. You can talk about being on the slow bus, but I haven't figured out the supposedly much improved 'Noise Reduction' function of later versions. Anyway, it's actually fairly simple when you know how.
1) Open Audacity.
2) Load your audio file.
3) Select a small section of the file that contains the noise you want to get rid of.
4) Go to the Noise Removal function in the Effects tab.
6) Go back to your file and select the whole thing.
7) Go back to Noise Removal and hit 'remove noise'.
Bob's your uncle! It takes a while to do fairly big files and then to save them as well - but I have achieved some very good results doing this. It's prudent, if you have any control over how the file is being recorded, to record a few seconds of ambient noise - meaning just room noise without talking. Then, when you get to Audacity, this will be the section you select for your noise profile, and all similar sounding noise will be removed from your file.